MJO Phase Speed and Blocking

The phase speed of the MJO might be regulated by many different factors. Previous works have suggested that moist processes govern the phase speed. Yet, convection and rainfall tend to be less intense in MJO events propagating more slowly than 5 ms⁻¹ than for MJO events moving at around 5 ms⁻¹. This presentation reflects on a dynamical feedback that might influence MJO phase speed: Rossby wave breaking and blocking. A wavelet filter is applied to extract time series characterized by selected zonal wavenumbers and frequencies at select equatorial base longitudes. Results show that anomalies of active convection characterized by wavenumber 2 (the dominant scale of MJO convection over the warm pool) are associated with meridional potential vorticity (PV) gradients across the tropics to the east of the active convection that are near climatology for events moving east at 5 ms⁻¹. These gradients are much weaker for slower events. The slowest phase speed events have almost no meridional PV gradients across the tropics between the mean latitudes of the subtropical jet streams, suggesting that jet exit regions occur immediately east of the deep convection, dumping mass in the upper troposphere over the region of suppressed convection. In the absence of PV gradients, synoptic to planetary scale waves moving into that environment break or cease to propagate linearly.